1. Field of the Invention
The present invention relates to a high-power amplification circuit for amplifying a high frequency signal, more particularly, a high-power amplification circuit including a circuit for switching a bias current to be supplied to the high-power amplifier.
2. Description of the Related Art
In a high-power amplifier using a bipolar transistor, as a collector current is greatly influenced by the variation in temperature, a bias current of the high-power amplifier is supplied by, for example, a current mirror circuit in which transistors are diode-connected. On the other hand, an amplifier which is linear and highly efficient at an output level of a wide dynamic range, is implemented by setting the bias condition to the B class (a scheme of supplying the bias current until the collector current stops flowing when the input of the high-power amplifier is zero) and reducing the bias current. Actually, however, as distortion caused by variation in gain becomes great due to non-linearity of an element, linearity of gain is maintained at the output level of the wide dynamic range in a manner such as the AB class that allows the bias current to flow to some extent.
Furthermore, a high-power amplifier having a wide dynamic range and using a CDMA (Code Division Multiple Access) system, which is adopted by, e.g., a mobile telephone, uses a low control voltage. Therefore, a high-power amplification circuit is therefore required to highly efficiently operate in its outputting operation. Thus, in order to achieve such a highly efficient operation, the high-power amplifier is operated with a bias current having a small value, such that a power of an output signal is reduced.
FIG. 5 shows an example of a bias circuit employing an emitter follower. In this circuit, a power supply voltage Vcc is applied to a collector of a transistor Q10. Also, a reference voltage Vref is applied to a base of the transistor Q10 through a resistor R10. An emitter of the transistor Q10 is grounded through a resistor R11. In addition, diode-connected transistors Q8 and Q9 are connected in series between the base of the transistor Q10 and the ground.
In a high-power amplification circuit using such a bias circuit, however, the transistors are stacked at two stages. Thus, it is impossible to sufficiently compensate for the variation in the bias current to the influence of the variation in temperature unless the reference voltage Vref is increased to be much higher than the double of an ON-state voltage of the transistors. On the other hand, the increase in the reference voltage Vref is a serious problem in a system such as a cellular phone having a low control voltage. Especially, when the high-power amplification circuit is operated with a bias current having a small value, such that the power of the output signal is reduced, the variation of the bias current which is caused by the temperature change is a problem for the high-power amplification circuit.
Various types of bias circuits for stabilizing the amount of a bias to be applied are known as a related technique (see Jpn. Pat. Appln. KOKAI Publication No. 2003-58262).